First, it's important to know that Christina Rossetti (1830-1894) was a devout Anglo-Catholic and her life was completely centered on religion. Her religion was so important to her, in fact, that she rejected two suitors--one for being a Roman Catholic (instead of an Anglo-Catholic) and one for expressing agnostic beliefs (that is, unsure whether there is a God).
"Up-Hill" is informed by both Rossetti's view of life as a religious journey and the world view expressed by upper-class Victorians, best expressed in the phrase, "Work, for the night is coming." In other words, do your duty while you're alive so that you can participate in the afterlife.
Rossetti frames the poem as an question-and-answer conversation between someone who is journeying through life and someone who has presumably already made it through life to the afterlife and has the answers. Typical of both a traditional religious and Victorian view of life, the answer to the first question--is the road uphill all the way--is, yes, life is hard right up to the end, and each day's journey has no shortcuts.
"Up-Hill" is an aggressively hopeful poem, for when the traveller asks if there will be a place to rest along the way, the answer, of course, is yes--"you cannot miss that inn." Not only will the traveller find rest but also company ("those who have gone before") along this journey--an important comfort for those who may feel isolated in their daily struggles.
At the end of this journey, when "travel-sore and weak," the traveller naturally wonders if there is ultimate comfort. Again, Rossetti reminds the traveller of the importance of work in the context of eventual rest--if you work all your life diligently, the reward for that work will be "beds for all who come."
We have, then, a very conventional view of life based on the combined values of religion and work.
The poem is awesome...